Low-floor commercial vehicles, such as city buses, have rear axles supporting and driving the vehicle. As largely described in prior art, such axles are typically offset axles or inverted portal axles in order to provide a vehicle with a low floor. An inverted portal axle is a rigid axle having a gear box at each longitudinal end and where the axle shafts are offset vertically relative to the wheel axis. In such axles, the typical centered differential is reduced in size and moved on one side. However, the combined weight of the axle housing, the live axle, the differential and a part of the drive shaft contribute to an undesirably high unsprung weight. Moreover, the torsional stiffness of the tubular nature of these axles limits the suspension linkages to an arrangement which reduces the available space in the aisle between wheel wells.
Alternatively, bowed dead axles such as deDion type suspension systems provide the desired torsional flexibility and also provide the advantage of slightly lowering the floor of the vehicle's body by using a body-mounted differential. However, such systems have not been used on heavy vehicles, such as buses and trucks, because of the sever torsion stresses when resisting vertical displacement of the drive wheels. Eliminating such torsional deflection by increasing the size and the weight of the bowed dead axle is both cost and design prohibitive. Another issue with the deDion type suspension is the prominence of the central differential which is not addressed.
U.S. Pat. No. 5,188,195 introduced a concept of a straight dead axle eliminating the torsional stress by introducing conical bearings supporting the dead axle. If this concept addresses some issues of deDion type suspension systems for heavy duty vehicles, it still makes use of a prominent centered body-mounted differential.
Twist beam axles are also known from the art and typically used on the rear wheels of front-wheel drive cars and particularly of small cars. The twist beam axle typically includes a transversal beam that extends between two trailing arms. The right and left trailing arms are pivotally supported on the vehicle body at their front ends. The transversal beam is generally U-shaped, V-shaped or H-shaped in cross-section, or constructed from hollow pipe with closed and deformed section, and may include a stabilizer bar to tune the torsional stiffness. For a rear-wheel drive vehicle, such a configuration is generally a problem in that the twist beam interferes with others members such as a drive shaft. To overcome this problem, some variations include a bent portion in the beam to make room for the drive shaft; however this affects bending stiffness. The rear drive axle of a heavy commercial vehicle significantly increases the challenge. Higher bending strength and stiffness (vertically and horizontally) are required without sacrificing flexibility in torsion. To increase bending strength and stiffness, the area moment of inertia of the beam cross-sectional area has to be increased. However, when doing so, the beam needs to be increased in length to keep its flexibility in torsion. On typical heavy trucks and city buses, the rear axle comprises a pair of double tires which significantly reduce the available space between wheels limiting the torsion capability of a twist beam.
Twist beam axles are however desirable for rear and front axles of heavy vehicles such as city buses since it may contribute to minimize torsion stresses on the vehicle body, and they are cost effective and durable.